Optical recording substrate, optical recording medium, and manufacturing method thereof

ABSTRACT

A method of manufacturing an optical recording substrate, comprising the steps of:
         contacting the surface having pit or land/grooves of a stamper having the pit or land/grooves to an organic polymer sheet having a glass transition temperature of 120 to 190° C., a single-pass birefringence retardation of +10 nm to −10 nm and a thickness of 0.35 mm or less under reduced pressure; and   thermally pressing them. According to this method, an optical recording substrate which can transfer a stamper pattern to a thin organic polymer sheet easily and thoroughly can be manufactured.

TECHNICAL FIELD

The present invention relates to an optical recording substrate, anoptical recording medium comprising the substrate and a method ofmanufacturing the substrate. More specifically, it relates to an opticalrecording substrate which can be used with an optical head having a highnumerical aperture and light having a short wavelength, an opticalrecording medium which comprises the substrate and is capable ofhigh-density recording, and a method of manufacturing the opticalrecording substrate.

BACKGROUND ART

Polycarbonate resins and polymethyl methacrylate resins have been widelyused as substrate materials for optical recording such as optical disksand optical cards because they are excellent as optical materials. Outof them, polycarbonate resins are widely used as substrate materials foroptical disks as they have excellent transparency, heat resistantstability and toughness.

In recent years, the recording density has been increasing due totechnical progress typified by an increase in the capacity of an optical(opto-magnetic) recording disk, the development of DVD and thedevelopment of a blue laser. The thickness of a disk substrate isreduced from 1.2 mm for CDs to 0.6 mm for DVDs, and a thinner disksubstrate is required to increase the recording density of an opticaldisk. However, in the production of an optical disk substrate having athickness of 0.35 mm or less by injection molding, it has been difficultto obtain a substrate which is satisfactory in terms of the filling of aresin into the periphery of a substrate, thickness uniformity on innerand outer sides, and transferability of pits and grooves because theresin is cooled very quickly. Although it is possible to obtain a 0.3mm-thick substrate which is satisfactory in terms of thesecharacteristic properties by changing a mold and molding conditions, thevalue of birefringence becomes extremely large as molecular orientationby a flow of the resin at the time of molding is hardly eased. Theoptimization of the production conditions has its limit in the reductionof birefringence. To reduce the birefringence, amorphous polyolefinshave been proposed by Zeonex, Zeonor (Japan Zeon Co., Ltd.) and Arton(JSR Corporation). Since these polyolefins have a small photoelasticcoefficient, a 1.2 mm-thick polyolefin substrate having lowerbirefringence than a polycarbonate substrate can be obtained. However, a0.6 mm-thick polyolefin substrate has a birefringence about ½ that of apolycarbonate substrate and a 0.3 mm-thick polyolefin substrate hasalmost the same high birefringence as a polycarbonate substrate. Whenthe thickness is 0.1 mm, it is difficult to obtain a substrate having apredetermined outer diameter by injection molding and the stiffness ofthe substrate is reduced, whereby it is difficult to remove thesubstrate from a stamper and take it out by a robot.

In contrast to this, a method of transferring a stamper pattern to afilm by heat and pressure by contacting the film to the stamper has beenproposed. JP-A 1-113224 (the term “JP-A” as used herein means an“unexamined published Japanese patent application”) proposes a method ofpressure molding a thermoplastic resin film or sheet while a DC field isapplied to the film or sheet at a temperature higher than the glasstransition temperature of the thermoplastic resin material and disclosesthat birefringence nonuniformity is reduced by the above method. JP-A4-270633 proposes a method of heating with a film-like heater sandwichedbetween a heating plate and a stamper and a method of letting pass acombination of the stamper and a thermoplastic resin between heating andpressure rollers to improve nonuniformity in birefringence. According tothese methods, temperature elevation and cooling times are shortened toimprove productivity but a stamper pattern is not partly transferred.Even if this defect cannot be observed with the naked eye, a fine defectas large as several nanometers to several micrometers can be observedthrough a microscope or AFM (atomic force microscope) and becomes adefect of a medium. A continuous transfer method which uses rollers andis different from these methods using a leaf type press is alsoproposed. JP-A 5-269845 proposes a method of transferring a pattern bysandwiching a molten resin sheet between a roll stamper having apreformat pattern on the surface and a mirror surface roller. As theresin sheet is continuously supplied in this method, the productivity ishigh and the number of partial transfer defects as described above isrelatively small. However, force applied to the resin sheet in thelongitudinal direction differs from that in the transverse direction andit is extremely difficult to maintain dimensional uniformity. JP-A11-345436 proposes a method of correcting the position and shape of aresin sheet relative to a stamper in anticipation of its deformation.However, the size and shape of the sheet are slightly changed by windingconditions and pressing conditions and it is difficult to control thesecompletely.

The optical recording density is expected to become higher and higherfrom now on and the appearance of an optical recording substrate whichcan be used with a laser light source having a short wavelength and anoptical head having a high numerical aperture is desired. Particularly,it is urgently desired to increase the recording density of an opticaldisk having a high access speed and capable of high-density recording byusing laser light having a short wavelength. A 0.35 mm or less thicksubstrate having transferred grooves and pits is the most promising torealize that. In the prior art, a substrate having the above thicknessand satisfying the above requirements cannot be obtained by injectionmolding or press molding.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method ofmanufacturing an optical recording substrate which solves the aboveproblems, particularly an optical disk substrate.

It is another object of the present invention to provide a method ofmanufacturing an optical recording substrate, which can transfer astamper pattern to a thin organic polymer sheet easily and thoroughly bya thermal pressing method.

It is still another object of the present invention to provide a methodof manufacturing an optical recording medium capable of high densityrecording using an optical recording substrate manufactured by the abovemethod.

It is a further object of the present invention to provide an opticalrecording substrate and an optical recording medium obtained by themethods of the present invention.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a method ofmanufacturing an optical recording substrate, comprising contacting thesurface having pit or land/grooves of a stamper having the pit orland/grooves to an organic polymer sheet having a glass transitiontemperature of 120 to 190° C., a single-pass birefringence retardationof +10 nm to −10 nm and a thickness of 0.35 mm or less under reducedpressure and thermally pressing them.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by a method ofmanufacturing an optical recording medium, comprising forming areflection film and/or a recording film on the surface to which the pitor land/grooves of a stamper have been transferred of an opticalrecording substrate obtained by the above method of the presentinvention.

According to the present invention, thirdly, the above objects andadvantages of the present invention are attained by an optical recordingsubstrate and an optical recording medium obtained by the above methodsof the present invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram for explaining the production process of an opticalrecording substrate used to carry out the method of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

A description is first given of the organic polymer sheet.

The thickness of the organic polymer sheet in the present invention is0.35 mm or less, preferably 0.35 to 0.001 mm. The thickness of asubstrate existent in the optical path up to a reflection surface ispreferably as thin as possible because the aberration of light when therecording surface (reflection surface) of an optical recording medium isinclined by warp becomes large depending on the distance between theobject lens of an optical pick-up and the recording surface (reflectionsurface) of the optical recording medium. Although the thickness of aDVD which is put on the market is 0.6 mm, a substrate (lighttransmission layer) having a thickness of 0.3 mm or 0.1 mm is needed.The present invention can be used to manufacture the thin substrate.

The material of the optical recording substrate of the present inventionmust have a glass transition temperature of 120 to 190° C. Since anoptical disk is often used in a car at a high temperature, a heatresistance test at 110° C. is required for the optical disk. When theglass transition temperature of the optical disk is low, not only thethermal deformation of appearance but also the deformation of a formatpattern of pits or lands/grooves caused by the heat of a laser at thetime of recording or reproduction or/and the application of a high-powerlaser beam to a phase change optical recording medium at the time ofinitialization (initial crystallization) may occur. From this point ofview, the glass transition temperature is preferably as high aspossible, for example, 120° C. or higher, more preferably 130° C. orhigher. When the glass transition temperature is too high, it isdifficult to transfer the pit or land/grooves of the stamper to thesheet at the time of molding, resulting in a transfer failure or largebirefringence.

One of the big features of the present invention is that the single-passbirefringence retardation of the organic polymer sheet used as amaterial before molding is +10 nm to −10 nm. The term “birefringenceretardation” means retardation when light having a wavelength of 633 nmis incident vertically on the surface of the sheet (vertical incidence).When the retardation is in the range of +10 nm to −10 nm, satisfactoryelectric properties are obtained. Retardation differs according to thewavelength of laser light used for the recording and reading of anoptical recording medium. The retardation value with laser light havinga wavelength of 400 nm differs only about 2% from the retardation valuewith laser light having a wavelength of 633 nm. If an optical recordingmedium for recording and reading with laser light having a wavelength of420 nm satisfies this range, satisfactory electric properties areobtained. Examples of a sheet material which satisfies the abovecondition include polycarbonate resins and amorphous polyolefin resinssuch as Zeonex and Zeonor (of Nippon Zeon Co., Ltd.) and Arton (of JSRCorporation). A copolymer of polystyrene having opposite birefringenceto that of a polycarbonate and polycarbonate may also be used. Theannealing of a sheet manufactured by an extrusion method is effectivebecause its absolute value of birefringence is large or itsbirefringence in a lengthwise direction differs from its birefringencein a transverse direction. The PURE-ACE (trade name, manufactured byTeijin Limited) which is a cast polycarbonate film is particularlyexcellent because it has a small birefringence and almost no filmthickness distribution.

At least one side in contact with the stamper of the organic polymersheet used in the present invention is preferably roughened. Asdescribed above, in the conventional leaf type press method, not only anuntransferred portion as large as 1 mm to several centimeters which canbe observed with the naked eye but also a very small untransferredportion which can be detected by a microscope or AFM are existent and ithas been difficult to completely solve this problem by changing onlypressing conditions. The inventor of the present invention has conductedintensive studies on this problem and has found that air remainingbetween the stamper and the organic polymer sheet is the main cause ofthis. When the organic polymer sheet is placed upon the stamper, theboth are partially closely adhered to each other and air remainingbetween them may not be completely removed. Particularly when thepressing temperature is set higher than the glass transition temperatureof the organic polymer sheet, partial fusion between the contactportions of them occurs and air is not removed from an area surroundingthese contact portions even by carrying out evacuation for a long timeafter that. To solve this problem, air existent between the organicpolymer sheet and the surface having pit or land/grooves of the stamper(may be referred to as “information surface” hereinafter) must becompletely removed before the both are contacted to each othercompletely, or the pressing temperature must be set to room temperatureor lower than at least the glass transition temperature to assemble themtogether, and evacuation must be fully carried out before the pressingtemperature is raised. However, in the former case, the apparatusbecomes complex and in the latter case, the production speed becomeslow. To improve this, it is effective that the surface of the organicpolymer sheet in contact with the information surface of the stamper beroughened or embossed and the pressure be reduced before pressing sothat the remaining air is easily escaped. If air remains on the surfaceopposite to that surface, the substantial contact pressure to be appliedto the stamper may change and affect transferability. Therefore, theopposite surface is preferably roughened or embossed as well. Theroughened surface may have any form but when it contacts the stamper, anair layer is preferably continuous. In this sense, projections on thesurface of the sheet are more preferred than depressions. For example,the sectional form of each projection may be square, rectangular,circular, triangular or trapezoidal with a flat top. The height of eachprojection is suitably selected but when it is too small, the effect ofimproving the escape of air is small. When it is too large, part of theprojection form remains after thermal pressing disadvantageously. It ispreferred that before pressing, the projections should not contact thestamper completely and after pressing, the projections should disappearcompletely. From this point of view, the height of each projection ispreferably 1 to 100 μm, more preferably 1 to 20 μm. The method ofroughening the surface is not particularly limited. For example, asubstrate having an uneven surface such as a woven or knitted stuff iscontacted to the sheet to transfer its surface uneven form to the sheetby thermal pressing. To make simple the top form of each projection onthe sheet, it is preferred not to have the resin of the sheet enterbetween the fibers of the woven or knitted stuff. To this end, the wovenor knitted stuff is slightly impregnated with a suitable resin to fillit between the fibers but retains an uneven surface. Since filling ispreferably carried out by thermal pressing, the resin to be filledpreferably has heat resistance. A fluororesin such as Teflon (registeredtrademark) is preferably used because it has excellent releasabilityfrom a sheet. For example, a glass fiber sheet impregnated with Teflon(registered trademark) is preferably used.

As the organic polymer sheet needs to transmit laser light for recordingand reading information, it preferably has a high transmittance of 85%or more, for example.

The optical recording medium has guide grooves for a light beam forrecording and reading information and pits for recording positioninformation, information on the revolution of a disk with a drive andrecording and reading conditions for signals. In the method of thepresent invention, the optical recording substrate is manufactured bytransferring these forms to the organic polymer sheet by thermallypressing the information surface of the stamper under reduced pressure.The reduced pressure is preferably about 0.3 atm. (3×10⁴ Pa) or less toremove air from the system in order to assist transfer. It is morepreferably about 0.1 atm. because the apparatus is airtight. The term“thermal pressing” means the application of pressure under heating. Thetemperature used for thermal pressing is preferably 5 to 50° C., morepreferably 15 to 35° C. higher than the glass transition temperature ofthe organic polymer sheet. When the temperature is lower than the aboverange, satisfactory transfer is hardly effected and when the temperatureis higher than the above range, a problem such as the deformation of theorganic polymer sheet readily occurs. For example, in the case of aresin having a glass transition temperature of 145° C., the temperatureis preferably 160 to 180° C. The preferred pressure to be applied whichchanges according to the used resin, temperature and other conditionsis, for example, 6 to 16 MPa. When the pressure is lower than the aboverange, satisfactory transfer is hardly effected and when the pressure ishigher than the above range, it may be difficult to release the organicpolymer sheet when taken out after thermal press molding.

To apply pressure to the organic polymer sheet/stamper informationsurface uniformly at the time of thermal pressing is important in orderto prevent the formation of an untransferred portion. When there is aproblem with the parallelism of the press (between upper and lower presstables) and the smoothness of contact surfaces, pressure is not applieduniformly. Thermal pressing by sandwiching the organic polymer sheet andthe stamper between metal plates is preferred and often carried out. Thesmoothness of the metal plates is not always satisfactory. In this case,pressure is hardly transmitted uniformly and a transfer failure area maybe formed. To minimize this influence during the thermal pressingoperation, a cushion material is preferably used. From the viewpoints ofelasticity and heat resistance, silicone rubber or fluororesin-basedrubber is preferably used as the material of the cushion material. Whenthe thickness of the cushion material is too large, heat conductivitybecomes small and when the thickness is too small, the effect ofapplying pressure uniformly decreases. Therefore, a cushion materialhaving a thickness of about 0.5 to 2 mm is preferably used. It iseffective to use this cushion material between the press table and thestamper or between the press table and the organic polymer sheet,preferably both.

The size of the optical recording substrate manufactured by the methodof the present invention is not particularly limited. When the opticalrecording substrate is an optical disk substrate, it is generally 30 mmto 300 mm in diameter and preferably doughnut-shaped with a center holehaving a diameter of about 15 mm.

The optical recording substrate of the present invention can bemanufactured by modifying conventionally known thermal press moldingequipment. To obtain the reduced pressure, for example, an areasurrounded by a heat resistant material having elasticity such assilicone rubber may be evacuated by a vacuum pump. A problem of theconventional leaf type thermal press that cooling and heating take timecan be solved by using a working stage which enables four stepsconsisting of the step of taking out each substrate and setting anorganic polymer sheet, the step of preheating, the step of thermalpressing and the step of gradually cooling to be automatically andcontinuously carried out in one cycle as shown in FIG. 1 in Example,thereby improving productivity.

The optical recording substrate to be manufactured by the method of thepresent invention is a substrate used for optical cards and opticaldisks, particularly rewritable disks such as optical magnetic recordingmedia, phase change recording media, media using a dye which can writeonce (to be called “-R”) and media in which signals are originallyrecorded in the form of pits (called “-ROM”).

A recording layer and/or a reflection layer and optionally a protectivelayer or protective resin layer are formed on these optical recordingsubstrates according to purpose. If necessary, two of the abovelaminates are laminated and used as an optical recording medium, forexample, optical disk. The recording layer and/or reflection layerare/is formed on the surface on which the pit or land/grooves of thestamper have been transferred of the optical recording substrate. In anoptical disk which reads information by using a laser having awavelength of 420 nm or less called “blue recording media” or recordsand reads information, a thin substrate is in keen demand and theinfluence of birefringence is more important. Therefore, the effect ofthe present invention is large.

The optical recording substrate in the present invention is a thinsubstrate for high-density optical recording which has pits and groovesand is advantageous in shortening a writing and reading wavelengthrequired for future high-density recording and increasing the numericalaperture of an optical head. Particularly, it is an optical disksubstrate used for high-density recording.

EXAMPLES

The following examples are given to further illustrate the presentinvention. The present invention is not limited to these examples.

Example 1

An optical recording substrate was molded with the NIC200 moldingmachine of Nissei Jushi Kogyo Co., Ltd. This molding machine comprisesfour stages as shown in FIG. 1: (1) setting a sample (sheet) and takingout a substrate (press 0), (2) preheating press (press 1), (3) heatingpress (press 2) and (4) cooling press (press 3). A laminate consistingof a stamper and an organic polymer sheet was fixed on a rotary board Rto be turned and moved every predetermined time while its interface waskept in reduced pressure. In the press 0, materials to be pressed suchas the stamper and the organic polymer sheet were laminated. Afterlaminated, the press 0 was covered, evacuation was started and theresulting laminate was turned and moved to the next stage. The materialsto be pressed were preheated by the press 1 and heated to apredetermined temperature by the press 2 to transfer the surface form ofthe stamper. Thereafter, the laminate was cooled and fixed by the press3 and the obtained substrate was taken out from the opening A of thepress 0.

As the organic polymer sheet was used a 100 μm-thick PURE ACE film(trade name; to be referred to as “PC film”, manufactured by TeijinLimited) which is a polycarbonate film manufactured by casting. Thisfilm had a glass transition temperature measured by DSC (differentialscanning calorimetry) of 160° C. and a single-pass birefringenceretardation of 7 nm. The stamper for blue laser recording had an outerdiameter of 138 mm (outer diameter of a recording zone: 118 mm) at agroove depth of 45 nm and a track pitch of 0.3 μm (land/grooverecording). The PC film was contacted to the information surface of thestamper and the resulting laminate was sandwiched between 1.0 mm-thickstainless steel plates (the contact surface of the PC film was a mirrorsurface) and set at the press 0 position of a sheet molding machine.After setting, evacuation was started, and the laminate was moved to thepreheating step after 10 minutes. Pressing was carried out at apreheating temperature of 150° C., a heating temperature of 170° C. anda pressure of 20 tons for a time of 1 minute. The cooling step wascarried out at a temperature of 20° C. for a time of 1 minute.

The obtained optical disk substrate was observed (1) with the naked eye,(2) by illuminating with a halogen lamp as a powerful light source, and(3) through an AFM (the SFA-300 atomic force microscope of SeikoInstruments Inc.) to check the transfer of grooves and the existence ofa fine defect. Five sheets manufactured under the same conditions wereobserved. They were satisfactory substrates which had no problem withtransferability when judged from the shapes of lands and grooves and thedepth of grooves obtained by observation through AFM and no surfaceabnormalities (partial clouding due to transfer abnormality) whenobserved with the naked eye.

It was confirmed when observed through a halogen lamp that the substratehad no problem.

The birefringence retardation of the organic polymer sheet aftertransfer was 6 nm which was slightly lower than that before the moldingof the sheet.

Comparative Example 1

A sheet was molded in the same manner as in Example 1 withoutevacuation. In this case, a transfer failure was observed in about 40%of the total area with the naked eye. It can be understood when comparedwith Example 1 that evacuation is necessary to effect satisfactorytransfer.

Example 2

The procedure of Example 1 was repeated except that one side of a PCfilm was roughened by contacting it to a glass fiber fabric impregnatedwith Teflon (registered trademark) using the above sheet molding machineand the evacuation time in the press 0 was changed to 1 minute. Thisroughened surface was contacted to the information surface of thestamper, and molding and evaluation were carried out in the same manneras in Example 1. In this case, although the evacuation time in the press0 was made shorter than in Example 1, good evaluation results wereobtained for all the five samples. It was thereby confirmed that a highyield could be obtained by roughening the surface of the PC film afterthe molding cycle was shortened to raise productivity.

When the thickness distribution of the film was measured at intervals of1 cm in both longitudinal and transverse directions, the average valuewas 100.2 μm and the standard deviation was 1.0 μm before molding. Whenit was measured at the same positions after molding, the average valuewas 100.5 μm and the standard deviation was 1.2 μm. It was therebyunderstood that the roughening of the surface did not have any influenceupon the thickness distribution after molding and a small thicknessdistribution was maintained.

Example 3

A sheet was molded in the same manner as in Example 1 except that apartially deformed stainless steel plate was used as the stainless steelplate and a 1.2 mm-thick silicone rubber sheet was inserted between thepress table and the deformed stainless steel plate (constitution:silicone rubber/stainless steel plate/PC film having one roughenedside/stamper/stainless steel plate). Five optical disk substrates weremolded and evaluated in the same manner as in Example 1. Good resultswere obtained in all the evaluations (1) to (3) of all the substrates.It was considered that the silicone rubber functioned as a cushionmaterial even though the stainless steel plate was slightly deformed sothat force was applied uniformly to all the contact surfaces of the PCfilm and the stamper.

Example 4

A 300 μm-thick polycarbonate sheet molded by a melt extrusion method wasused in place of the PC film used in Example 1. As the birefringenceretardation of this sheet was 31 nm, it was annealed at 165° C. for 20minutes before use. This treatment reduced the birefringence retardationof the sheet to 1.2 nm. The sheet molding conditions were the same as inExample 1. In this case, five substrates were evaluated in the samemanner as in Example 1. Good results were obtained in all theevaluations (1) to (3). The birefringence retardation after the moldingof the sheet almost remained unchanged at 1.1 nm.

Comparative Example 2

A sheet was molded in the same manner as in Example 4 except thatannealing was not carried out. The birefringence retardation aftermolding of the sheet was 23 nm.

Evaluation of Electric Properties

The substrates obtained in Examples 1, 3 and 4 and Comparative Example 2were punched into a doughnut shape having an inner diameter of 15 mm andan outer diameter of 120 mm, a phase change recording film which was alaminate consisting of inorganic thin films was formed on the groovesurfaces of the substrates by sputtering, and a polycarbonate diskhaving a thickness of 1.2 mm, an inner diameter of 15 mm and an outerdiameter of 120 mm was laminated on the laminate of inorganic thin filmsto manufacture optical disks. The constitutions of the disks are shownin Table 1.

TABLE 1 film constitutions of phase change optical recording mediaOptical disk substrate of the present invention 0.1 mm or 0.3 mm ZnSSiO₂ dielectric film  50 nm SiO₂ dielectric film  50 nm ZnS SiO₂dielectric film  25 nm GeSbTe phase change recording film  17 nm ZnSSiO₂ dielectric film  17 nm AlCr metal reflection film 100 nmUltraviolet light curable resin adhesion layer About 2 μm Polycarbonateboard 1.2 mm

The method of manufacturing the laminate of inorganic thin films bysputtering is as follows.

A ZnS—SiO₂ film (film obtained by sputtering a target of ZnS:SiO₂=80:20mol %) and an SiO₂ film were used as dielectric films. The recordinglayer was a GeSbTe alloy film (Ge:Sb:Te=2:2:5 atom %). The reflectionlayer was an AlCr alloy film (Al:Cr=97:3 atom %). These inorganic thinfilms were formed on a transparent substrate by magnetron sputtering.The used sputtering device was the in-line sputtering device of ANELVACorp. (ILC3102), and the target was an 8-inch diameter substrate whichturned on its own axis and revolved while a film was formed thereon. Thethickness of each film was adjusted by the sputtering time. Thethickness of the dielectric film was adjusted to ensure that reflectancebecame about 25% when it was illuminated to the surface with lighthaving a wavelength of 410 nm. The refractive index of the ZnS—SiO₂ film(wavelength of 633 nm) was 2.18, the refractive index of the SiO₂ film(wavelength of 633 nm) was 1.53, and the reflectance of the recordingfilm after crystallization (initialization) (wavelength of 410 nm) was7.2%.

The recording and reading properties of these optical disks wereevaluated as follows. The evaluation machine used was the DDU-1000 ofPalsteck Kogyo Co., Ltd. Owing to differences in the thickness of thesubstrate (sheet), an optical head having a laser wavelength of 400 nmand an NA of 0.85 was used in Examples 1 and 3 (thickness of 100 μm) andan optical head having a laser wavelength of 402 nm and an NA of 0.65was used in Example 4 (thickness of 300 μm). To check absoluteperformance and the transfer uniformity of the stamper form, the size ofa read signal and disk one-cycle variations in the read signal (envelopevariations of read signal) were evaluated.

Evaluation Conditions

Lands and grooves were measured at measurement radii of 28 nm (innerend) and 58 mm (outer end)

Linear speed of recording and reading=4.0 m/sec

Linear recording density=0.115 μm/bit (1–7RLL modulation recording)

Recording laser power=3.2 mW (Examples 1 and 3), 4.5 mW (Example 4)

Erasing laser power=1.5 mW (Examples 1 and 3), 3.2 mW (Example 4)

Reading laser power=0.4 mW (Examples 1 and 3), 0.5 mW (Example 4)

Evaluation Results

Table 1 shows CNR (dB) of a 1–7 modulated 3T read signal and one-cyclevariations (%) in the amplitude of the read signal. Optical disk mediaof Examples have a CNR of 46 dB which is said to be required for digitalrecording. The change rate of the read signal envelope was satisfactoryat 5% or less. The optical disk medium of Comparative Example 2 had alow CNR. Particularly, the CNR difference between the land track and thegroove track was large which seemed to be influenced by the largebirefringence. The change of output envelope was large.

TABLE 2 Evaluation results of recording and reading properties CNR (dB)Envelope variation (%) Inner end Outer end Inner end Outer end Example 1Land 49.8 49.0 4.1 4.8 Groove 50.1 49.4 3.8 4.1 Example 3 Land 51.2 50.22.5 3.1 Groove 52.3 52.1 2.8 3.8 Example 4 Land 49.8 48.7 3.0 4.2 Groove47.5 47.2 3.5 4.6 Comparative Land 46.8 45.9 5.1 5.3 Example 2 Groove43.6 43.0 6.2 6.2

1. A method of manufacturing an optical recording substrate, comprisingthe steps of: contacting the surface having pit or land/grooves of astamper having the pit or land/grooves to an organic polymer sheethaving a glass transition temperature of 120 to 190° C., a single-passbirefringence retardation of +10 nm to −10 nm and a thickness of 0.35 mmor less under reduced pressure; and thermally pressing them.
 2. Themethod of claim 1, wherein at least the surface in contact with thestamper of the organic polymer sheet is roughened.
 3. The method ofclaim 1 or 2, wherein the organic polymer sheet and the stamper arethermally pressed on a press table by inserting a cushion materialbetween the organic polymer sheet and the press table and/or between thestamper and the press table.
 4. A method of manufacturing an opticalrecording medium, comprising forming a reflection film and/or arecording film on the surface to which the pit or land/grooves of thestamper have been transferred of an optical recording substrate obtainedby the method of any one of claims 1 to
 2. 5. An optical recordingsubstrate manufactured by the method of any one of claims 1 to
 2. 6. Theoptical recording substrate of claim 5 which is a disk.
 7. An opticalrecording medium manufactured by the method of claim
 4. 8. The opticalrecording medium of claim 7 which is a disk.
 9. A method ofmanufacturing an optical recording medium, comprising forming areflection film and/or a recording film on the surface to which the pitor land/grooves of the stamper have been transferred of an opticalrecording substrate obtained by the method of claim
 3. 10. An opticalrecording substrate manufactured by the method of claim
 3. 11. Theoptical recording substrate of claim 10 which is a disk.